The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to rf coil systems for applying an excitation field to a subject under examination.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
At high fields (>1.5T) wavelength effects cause the transmit RF field (B1 field) in the human body to be inhomogeneous. In head imaging, the B1 field is typically highest in the center of the head, and significantly lower in the temporal lobes and cerebellum. At 7 Tesla, the B1 field in the temporal lobes is typically about 50% of that in the center of the head. This causes variations in contrast over the head in MR imaging, and results in lower SNR in regions with low B1 field. Body imaging at 3T and above suffers from similar inhomogeneities in the B1 field.
There is at present no simple solution to the problem of B1 inhomogeneity in high field MRI. Various methods have been proposed and demonstrated to various degrees. One approach proposed by researchers from Siemens involves using a separate surface coil tuned off-resonance (not detuned, but tuned to a higher frequency) which was placed on the abdomen in a 3T scanner to increase the B1 field in an area where it was otherwise relatively low. Placing bags of water or ultrasound gel around the head has been shown to change the B1 field within the head. Controlling the phase and amplitude of individual rungs in a birdcage type transmit coil has been shown in modeling to change the B1 field and make possible greater homogeneity, but practical demonstrations have not fully realized the theoretical results. Shaped RF pulses when applied concurrently with appropriate magnetic field gradients can be used to compensate for B1 inhomogeneity, but the pulses are very long and not practical for most uses. The use of accelerated transmit techniques (sometimes referred to as Transmit SENSE) with a transmit array reduces the length of the shaped RF pulse to a more practical level, and this is currently a topic being studied by many groups. Transmit array designs to date have largely consisted of a number of coil elements on a cylindrical form arranged azimuthally around the head with no distribution of elements along the Z-direction.